Retirement Plans and Other Postretirement Benefits - Summary of Retirement Plans and Other Postretirement Benefits (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Change in plan assets:
Fair value of plan assets at beginning of year	$ 51,607,000
Fair value of plan assets at end of year	55,366,000	$ 51,607,000
Retirement Plans [Member]
Change in benefit obligation:
Benefit obligation at beginning of year	53,685,000	46,868,000
Service cost	477,000	436,000	$ 560,000
Interest cost	1,692,000	1,905,000	1,760,000
Actuarial loss	6,423,000	7,104,000
Benefits paid	(2,846,000)	(2,628,000)
Benefit obligation at end of year	59,431,000	53,685,000	46,868,000
Change in plan assets:
Fair value of plan assets at beginning of year	51,607,000	45,379,000
Actual return on plan assets	6,306,000	8,696,000
Benefits paid	(2,846,000)	(2,628,000)
Corrective payment	140,000	0
Employer contribution and non-qualified benefit payments	159,000	160,000
Fair value of plan assets at end of year	55,366,000	51,607,000	45,379,000
Funded status	(4,065,000)	(2,078,000)
Unrecognized net actuarial loss	24,526,000	22,433,000
Net amount recognized	20,461,000	20,355,000
Other Post Retirement Benefits [Member]
Change in benefit obligation:
Benefit obligation at beginning of year	3,069,000	2,283,000
Service cost	109,000	67,000	88,000
Interest cost	96,000	94,000	92,000
Actuarial loss	377,000	731,000
Benefits paid	(111,000)	(106,000)
Benefit obligation at end of year	3,540,000	3,069,000	2,283,000
Change in plan assets:
Fair value of plan assets at beginning of year	0	0
Actual return on plan assets	0	0
Benefits paid	(111,000)	(106,000)
Corrective payment	0	0
Employer contribution and non-qualified benefit payments	111,000	106,000
Fair value of plan assets at end of year	0	0	$ 0
Funded status	(3,540,000)	(3,069,000)
Unrecognized net actuarial loss	989,000	638,000
Net amount recognized	$ (2,551,000)	$ (2,431,000)